Storage tanks for oil and other liquid hydrocarbon products are typically provided with a floating roof. The floating roof moves vertically on the surface of the liquid product to prevent the escape of harmful vapors. A typical floating roof for covering a liquid product in a storage tank includes a deck whose peripheral side edges substantially conform to the horizontal cross-sectional shape of the storage tank. The roof has a vertical thickness and is provided with one or more seals extending between the outer periphery of the floating roof and the inner wall of the tank. The roof floats on top of the stored liquid product and rises and falls as the amount of the liquid product increases and decreases. It is known to provide a drain pipe extending vertically through the top surface of deck to drain water accumulated atop the roof and to provide a passage or conduit for the drained water from the surface of the roof to pass through the stored liquid and accumulate at the bottom of the tank where it is eventually drained from the tank.
During seasonal or other periods of heavy rains the amount of water that accumulates over the area of the storage tank roof or cover can be substantially greater than that which can be removed by the centrally positioned drain. The water will therefore rise and can inundate the drain sealing mechanism. Also, the presence of debris carried by the accumulating water can interfere with proper operation of complex roof drain apparatus known to the prior art.
A further problem can occur when rainwater accumulates on the roof and its effective weight increases to a point where it can overcome the buoyancy of the roof, eventually forcing it into the liquid in the tank. It is therefore important to provide an effective and reliable means for draining water from the roof so that it does not exceed a pre-determined depth and weight.
Various approaches have been proposed for draining water from storage tank floating roofs. For example, U.S. Pat. No. 2,560,586 to Michaels discloses a floating roof drain which drains water collected over a valve of the floating roof which closes and opens a drain passage. The weight of the water collected over the valve pushes down a cover against the buoyancy force of a float connected to the valve to allow the water collected to flow from the roof. The valve is again closed when the depth of the water on the roof decreases until it is no longer sufficient to hold the valve open against the buoyancy force of the float.
U.S. Pat. No. 2,913,138 to Swick describes floating covers for tanks in which a drainage device is located at a low point of the roof structure in a deck. The drainage device comprises a cylindrical sump, a sump bottom formed with a shallow depression which constitutes a downward flow passage, a cylindrical neck extending from the bottom of the sump and an annular float member loosely surrounding the sidewall of the neck. A mercury seal is provided on the bottom, and the float member rests on the bottom of the pool of mercury. Accumulated water in the sump buoys up the float member to interrupt the mercury seal and thereby provide a conduit for water to drain across the surface of the pool.
U.S. Pat. No. 3,883,032 to Fisher discloses an automatic drain valve for a floating roof which includes an orifice and a larger disk located under the orifice. A float attached to the disk allows the valve to be biased closed and water gathered on top of the roof will open the valve which permits the water to drain through the roof into the tank beneath to join the body of liquid product with the tank.
Japanese Patent No. JP5077883 to Kunio discloses an emergency drain device for a floating roof in which a drain pipe runs through a deck up to stored liquid in a tank and a float, which floats on accumulated liquid on the deck and is on an upper end of the drain pipe, prevents vapor from flowing out of the drain pipe. A weight-attached guide member makes the float return to the upper end of the drain pipe.
Still other prior art patents disclosing aspects of floating roofs and drainage systems are U.S. Pat. Nos. 2,563,017; 2,664,5220; 4,134,515; 4,202,366; 3,944,113 and US 2007/0272692; however, these prior art patents do not disclose a system the same as or equivalent to the present emergency drainage system described below.